A common-drain amplifier, also known as a “source follower,” is a single-stage Field Effect Transistor (FET) amplifier topology commonly used in analog Integrated Circuit (IC) design. In many circuits, source followers may be employed as voltage buffers. Additionally or alternatively, source followers may also be used to transform impedances. For example, when a voltage follower is driven by a voltage source having high output impedance, the output impedance of the combination is equal to the output impedance of the voltage follower only, which is relatively small.
The inventors hereof have recognized, however, that conventional source followers have certain shortcomings. For example, a traditional source follower only supports output excursion to one of the supply rails—to the lower supply rail if N-type or to upper supply rail if P-Type. Previous attempts to implement source followers with rail-to-rail excursion disadvantageously involve reducing bandwidth and/or otherwise degrading transient performance of the circuit.